Not able to fill in a face between an open space
By the looks of your screenshot, you do not have at least 3 vertices selected. With only two vertices selected, F will only create an edge. To create the face, try Shift+Click-ing on each of the four faces and then pressing F to fill.

Other ways of creating a bridging face:


Alt+Click, then Shift+Alt+Click on the edges to select the "Edge Loop" on each side of the gap. This will select the two connected vertices. Once all four vertices are selected, press F.
Select all four vertices with Shift+Click. Press F3 and search for "Bridge Edge Loops" and press Enter.
Press B to box select and drag a box over all four vertices, selecting them. Press F to fill.
Change to "Edge Mode" ('2' on the number row) select both edges with Shift+Click and press F to fill. You can get back to "Vertex Mode" with '1' on the number row.


If at any time, you select the wrong vertices, double tap A (dbl-A) to deselect all.